Fixed position optical scanners are used for a number of different purposes. The most widely known use for such scanners is to detect bar code labels affixed to products sold by supermarkets or retail stores. Additional uses for such scanners include detection of bar codes on parts or packages being transported by conveyors through distribution centers, warehouses or manufacturing facilities.
In supermarkets, the bar code information is used primarily to identify products. Once a product has been identified, the system can retrieve a stored price for use in generating a customer receipt tape. The product information can also be used, secondarily, to track sales of particular items or to control inventory levels. In distribution centers, warehouses and manufacturing facilities, the bar code information is used primarily to control distribution of parts and packages.
Fixed position optical scanners typically include a laser, a beam deflecting component for deflecting the laser beam along different scan lines to produce a scanning pattern, a photodetector for sensing optical energy reflected from an item in the path of a scanning beam and a processor for extracting bar code information from the signals produced by the photodetector. The processor is normally a digital processor while the signals provided by the photodetector are analog. To convert the photodetector signals to a form suitable for use by the processor, fixed position optical scanners include an analog to digital converter for periodically sampling the analog signal and for converting each of the samples to a digital signal. The rate or frequency at which the analog signal is sampled is typically fixed.
Moving mirrors are widely used to deflect the laser beam to form scan patterns. The mirrors may oscillate or rotate to generate a moving scanning beam from a stationary laser beam. An auxiliary condensing lens is normally used in a fixed position optical scanner to focus a scanning beam at a known distance from the beam deflecting element. Most optical scanners have a fixed depth of field or range of distances on either side of the focal point within which a bar code may be successfully read. The frequency with which the analog signal is sampled is established, at least in part, at a level appropriate for the scanner's depth of field.
A relatively recent development in fixed position optical scanners has been the use of a rotating holographic optical element or disk to both deflect and focus a laser beam. A holographic disk usually consists of a transparent glass or plastic disk which supports a ring or annulus of holographic optical elements or facets, each of which occupies a sector of the ring. Each of the sectors may be generated using known off-axis holographic techniques. Depending upon the configuration of light beams used in generating the facet, that facet will deflect an incident laser beam along a specific scanning path while focusing it at a specific distance from the facet surface. By changing the beam configurations used in producing different facets, a holographic disk can be formed in which different scanning beams have different focal lengths. By using some facets to produce scanning beams with shorter focal lengths and other facets to produce scanning beams with longer focal lengths, the range of distances over which at least one scanning beam will be sufficiently focussed to read a bar code label will be greatly increased.
The fact that a set of scanning beams are sufficiently focussed over a greater range of distances does not necessarily guarantee that a bar code will be successfully read over that range of distances. The analog signal produced by the photodetector in a holographic disk scanner must be sampled, digitized and decoded as it is in any other scanner.
Analog to digital converters or other sampling circuits which operate at a given sampling frequency are not well suited for scanners which are expected to read bar codes over a considerable range of distances. Practical beam deflecting devices, including moving mirrors or holographic disks, produce scanning beams which sweep with substantially constant angular velocities independent of the focal length of the beam. The linear velocity of a given beam is, however, a function of both its angular velocity and of the radius from the origin of the beam to the point at which the linear velocity is being measured. For example, if a beam has a constant angular velocity, the linear velocity of a beam measured at a distance 40" from the origin of the beam will be four times as great as the linear velocity of the same beam measured at a distance of 10" from the origin of the beam.
If the frequency with which the analog photodetector signal is sampled remains constant, the linear distance between sample points will be four times as great at a point 40" from the beam deflecting element as it is at a point 10" from a beam deflecting element. For bar code labels which meet minimum acceptable size standards, the sample points for a bar code read at a considerable distance from the scanner may be too far apart to adequately define the bar code pattern.
One possible solution to the problem is to establish a sampling frequency high enough to provide an adequate number of samples for the smallest allowable bar code if read by the scanning beam having the longest focal length. The problem with this approach is that the processor and associated electronic circuitry must be capable of continually processing data delivered at this high sampling frequency. Since the processor usually performs other tasks concurrently with processing of data signals, the use of a constant high sampling frequency for reading bar codes "loads" the processor, reducing its overall efficiency in performance of other tasks.
Another solution is simply to limit the minimum and maximum acceptable reading distances to those in which a fixed sampling rate would adequately define a bar code label without being so high as to "load" the processor. Such a solution is clearly impractical since it runs counter to one of the primary reasons for using a holographic disk to begin with; namely, to achieve scanning of bar codes detected over a considerable range of distances from the disk.